1. Field of the Invention
This invention is in the field of apparatus used to detect the presence of ferromagnetic threat objects to prevent the objects from being transported into the vicinity of a magnetic resonance imaging (MRI) magnet.
2. Background Art
Large ferromagnetic threat objects can be devastating when subjected to the strong magnetic field of a magnetic resonance imaging magnet. Pipe wrenches, floor scrubbers, oxygen cylinders, and even gurneys have been attracted to the MR magnet, as if propelled by a rocket, with disastrous consequences. At least one tragic death has occurred when a steel oxygen cylinder became, in effect, a lethal weapon. The problem is compounded when one considers the fact that many new MRI magnets have a much higher field of 3.0 Tesla (30 KOe). It is, therefore, prudent to screen people for such objects to prevent possible accidents.
Common metal detector portals, such as those used in airports, detect any metal. Hence they produce many false positive readings arising from coins, etc., that are non-magnetic, and, therefore, present no danger in the MRI setting. Ferromagnetic detection portals are very useful for ferromagnetic threat detection relative to a person or object passed through the portal. Nevertheless, disadvantages are present. First, ferromagnetic detection portals tend to be quite expensive, as these generally contain sensing elements, and other elements, on both sides of the portal, and, thus, these portals may be beyond the budget of some MRI centers.
Second, the side structures of these portals, when taken together, consume a significant surface area. This can be a major problem in a compact MRI center, such as a mobile truck. Indeed, in most mobile trucks, many ferromagnetic portals simply will not fit because of lack of room.
Many portals which are fixed in size are too large, and thus unable to squeeze into the restricted available space. In addition, some portals are designed such that threats trigger an alarm only when the portal is manually turned on. A naive orderly or technician may then simply omit turning on the portal. When the magnet room is entered with the threat object, a disaster can occur.
Placing a ferromagnetic detection system on the door of the magnet room itself is a doubtful proposition at best. By the time the alarm is triggered, the threat is already within the magnet room and, therefore, subject to the large magnetic field and gradient of the MRI magnet. If detection occurs in such a system, it is simply too late.